This invention relates to actuators in particular ceramic actuators which may be electro-active, for example piezoelectric. In one aspect, this invention relates to a housing for such actuators which may be applied to a camera in which the actuator moves a lens holder. In another aspect, this invention relates to a suspension for a camera in which the actuator moves a lens holder. Both aspects may be applied to micro-cameras in portable data processing or communicating devices.
Piezoelectric and other electro-active benders made from ceramic base material such as lead zirconate titanate (PZT) are used in many applications. They are manufactured for example from multilayer (green) material and sintered at high temperatures into their final shape.
A variety of configurations for such actuators are known. Comparably large translation displacements have been recently achieved by using a structure of piezoelectric bender tape extending helically around an axis which is itself curved, as described, for example, in WO-0147041 or D. H. Pearce et al., Sensors and Actuators A 100 (2002), 281-286. Such devices are capable of exhibiting displacement in the order of millimetres on an active length of the order of centimetres.
Whilst the manufacturing of ceramic actuators is known, their applicability is limited due to the brittleness of the material they are made of. It would therefore be desirable to provide housing for ceramic actuators that reduces the sensitivity of ceramic actuators against sudden impacts as caused for example by a drop onto a hard surface.
As a separate matter, in recent years, with the explosive spread of portable information terminals called PDAs and portable telephones, an increasing number of products incorporate a compact digital camera or digital video unit employing a CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor) sensor as an image sensor. When such a digital camera or the like is miniaturized using an image sensor with a relatively small effective image-sensing surface area, its optical system also needs to be miniaturized accordingly.
To achieve focussing or zooming, additional drive motors have to be included in the already confined volume of such miniature cameras. Whilst most of the existing cameras rely on variations of the well-known electric-coil motor, a number of other actuators have been proposed as small drive units for the lens system. These novel drive units often include actuators of electro-active material, for example piezoelectric, piezoresistive, electrostrictive or magnetostrictive material, typically ceramic actuators.
Small electro-active actuators with comparably large translation displacements have been recently build using a structure of piezoelectric bender tape extending helically around an axis which is itself curved, as described, for example, in WO-01/47041 or D. H. Pearce et al., Sensors and Actuators A 100 (2002), 281-286. Such devices are capable of exhibiting displacement in the order of millimetres on an active length of the order of centimetres. They may be manufactured from multilayer ceramic base material such as lead zirconate titanate (PZT) and sintered at high temperatures into their final shape. The use of such actuators as drive motors for lens systems has been proposed in WO-02/103451.
As drive units adapt to the reduced volume of the compact camera designs, lens suspensions systems, which constrain the motion of the lens holder, have to co-evolve. Lens suspension systems suitable for miniaturized cameras, particularly for cameras driven by an electro-active transducer ideally have a low stiffness, resistive force or friction in direction of the desired motion and high stiffness in all other directions.
According to a first aspect of the invention, there is provided a housing for a ceramic actuator including a protective structure, preferably with compliant elements contacting the actuator, limiting the the range of motion of sections of the bender located distant from both fixed and moving terminals of the bender.
An actuator is typically operated with a fixed terminal or end section and a moving terminal or end section. The fixed terminal is attached to the housing or mounted on a base structure shared with the housing. The moving terminal of the actuator is the section of the actuator that displays the largest displacement relative to the fixed section of the actuator. It is seen as an important feature of the first aspect of the invention that the protective structure is adapted to limit the range of displacement of the actuator by contacting the actuator at one or more sections located between the two terminal sections of the actuator. Thus, the actuator is free to move, on actuation, within the range of displacement until it contacts the protective structure and references to contacting should be understood accordingly. Such a protective structure protects the actuator against a sudden impact, as caused for example by a drop onto a hard surface, by preventing excessive displacement of the actuator which would cause damage.
The actuator may be a linear actuator having a moving end section that describes a near linear motion.
Advantageously, the protective structure is placed outside the nominal range of displacement. The nominal range of displacement of the actuator is the displacement exhibited by the actuator during normal operating conditions. The limits of the nominal range of displacement define a surface or an envelope outside which the protective structure is located.
Advantageously, the protective structure follows the contour defined by the limits of the nominal range of displacement. For example, the protective structure may be arranged to have an approximately constant distance from the limits of the nominal range of displacement at different points along the actuator.
In one type of embodiment the protective structure includes one, two or more discrete elements adapted to contact the actuator, for example resilient members such as mechanical spring type structures for removal of energy from the actuator on impact. Suitable structures include for example resilient beams disposed along the protective structure.
In another type of embodiment the protective structure includes a continuous surface adapted to contact a section extending along the actuator.
Desirably, the portions of protective structure adapted to contact the actuator are compliant, for example by being resilient or by being formed by compliant material, so as to be capable of absorbing the kinetic energy of the actuator.
The protective housing of the invention may be readily manufactured for example by moulding of plastic materials. The protective structure may be advantageously moulded in one piece with the housing. The compliant layer, if provided, may be attached for example by glueing to the protective structure. Alternatively, the compliant layer or layers may be incorporated in the housing during manufacture by two-shot moulding, in which the housing and compliant layers are produced in different materials within the same mould. Resilient structures such as resilient beams may be advantageously moulded in one piece with the housing, forming the appropriate ramp contour. Alternatively, spring structures may be cut from thin metal sheet, for example by photo-chemical etching, bent to the appropriate shape if necessary, and then fixed into the housing, preferably by means of moulded locating pips in the housing.
The present invention is particularly advantageous used to house actuators capable of relatively large displacement, such as the actuators of the type mentioned above and disclosed in WO-0147041 or D. H. Pearce et al., Sensors and Actuators A 100 (2002), 281-286, because such actuators are particularly susceptable to damage from a sudden impact.
According to a second aspect of the invention, there is provided a camera including a support structure; a lens holder holding at least one lens; a suspension for mounting said lens holder on the support structure; and an actuator for moving said lens holder, wherein the suspension includes two link elements each pivotally connected to the support structure at one end and pivotally connected to the lens holder at the other end.
Such a suspension has a low stiffness, resistive force or friction in the direction of the desired motion and high stiffness in all other directions. It is thus suitable for miniaturized cameras, particularly for cameras driven by an electro-active actuator.
The suspension system is preferably a type of a four-bar linkage, in which the suspension further includes a first attachment member to which the first end of each link element is pivotally connected and which is attached to the support structure, and a second attachment member to which the second end of each link element is pivotally connected and which is attached to the lens holder. Such a type of suspension system can be formed from one continuous piece of material, preferably a plastics material, for example selected for example from a group including polypropylene, polyethylene and polyamide (nylon). Advantageously, the thickness of the link element tapers towards the pivotally connected ends such that the link element is thicker in the middle than in the immediate vicinity of the pivots or hinges.
Advantageously, the pivotally connected ends of the suspension extend along the circumference of the lens holder.
Preferably, the pivotally connected ends of the suspension extend along a length which exceeds a tenth, more preferably a third or even a half, of the diameter of the lens holder. This provides the advantage that, as compared to a suspension where this length is shorter, the suspension can sustain a higher torsional force without significant deformation.
In a preferred embodiment the actuator extends around the lens holder leaving a single gap with the suspension located in said gap. In this embodiment, the suspension supports the lens holder at just one side or relative to a cylindrical lens holder within just one sector. The sector, measured by connecting the end points of the longest pivot that is located at the lens holder with the center of the lens holder, is preferably less than 90 degrees. As a result, the lens holder is suspended at a quarter or less of its circumference—excluding the suspension effected by the actuator.
In some variants of the invention it may advantageous to limit the amount of rotational motion around the pivoting ends to less that 20 degrees, because, as a result, the lens holder's motion is limited to the equivalent maximum displacement which improves the protection of the actuator.